Neurogenic (vasovagal) syncope (VVS) is a significant medical problem. The faint is preceded by a Vasovagal Response (VVR), characterized by a sudden drop in arterial blood pressure (BP) and heart rate (HR). VVS and VVRs are diagnosed using a tilt test, a vestibular stimulus. Despite being relatively common, the neural basis of the VVR and the role of the vestibular system in inducing VVRs are not known. Moreover, there is no small animal model of VVRs. We recently showed that low frequency sinusoidal galvanic vestibular stimulation (sGVS) induces VVRs in anesthetized rats. As in humans, VVRs are also induced in rats by tilt. In the proposed research we will characterize the physiological properties of VVRs in rats, thereby establishing a useful small animal model of human VVRs. We propose that VVRs are induced by strong, repeated stimulation of the vestibulo-sympathetic reflex (VSR), which maintains blood flow to the brain during changes in head position relative to gravity. Aim 1 will characterize the transitions from normal VSR to VVRs using pulses and sinusoids of galvanic stimulation to show that VVRs occur when the VSR is appropriately stimulated. Nose up tilt and Translation-While-Rotating (TWR) will be used to determine the axes of stimulation that are most likely to produce VVRs. Additionally, the low frequency oscillations in HR and BP that are produced by sGVS and tilt reflect activation of Mayer waves, thought to be of fundamental importance in maintaining BP stability. In Aim 2 these oscillations will be modeled using a novel concept in which Mayer wave function is represented by a non-linear relaxation oscillator. In Aim 3, single unit neurophysiological studies in otolith-recipient portions of the caudal vestibular nuclei will identify the functional characteristics and physiological signature of VVR- related neurons. Together, the proposed research will establish a model of VVS, document how the vestibular system controls BP, and demonstrate that VVRs are adaptive compensatory responses that serve to re- equilibrate BP after major perturbations.